But I've seen others complain about "sites" not loading correctly and it seems there's a bug that needs to get fixed, but there is a workaround apparently.

''so far it appears to be an issue that only affects users who have turned off cleartype font rendering within their windows settings. so a workaround for the moment would either be to turn cleartype back on or disabling hardware acceleration in firefox: Firefox's performance setting''

I also have similar text rendering problems with Firefox 59.0.1. However, they also occur in many other websites, at least Reddit.

I have font smoothing disabled in Windows settings, and that apparently is a requirement for the problem or at least makes it much more likely to occur. (My setup is similar to yours, though: 64-bit Windows 10, and GeForce GTX 1060.)

The issue doesn't matter much to me because I use Vivaldi as my primary browser.

Another workaround that has been commonly suggested is disabling hardware acceleration in Firefox settings.